The present invention relates to an exhaust-gas turbocharger for an internal combustion engine.
An exhaust-gas turbocharger having a turbine with radial and semi-axial flow-inlet cross-sections in the inflow region of the turbine is described in German Published Patent Application No. 196 15 237. The flow-inlet cross-sections, between which a flow or guide ring, contoured in a manner favorable for the flow, is arranged in the flow-in region of the turbine, permit both a radial and a semi-axial flow against the turbine wheel. Disposed in the radial flow-inlet cross-section is a guide baffle having adjustable guide vanes by which the flow-inlet cross-section can be varied. The exhaust-gas backpressure, as well as the manner in which the exhaust gas flows in onto the turbine wheel can be influenced by the setting of the guide baffle, which means the output of the turbine and the performance of the compressor can be adjusted depending on the requirement and the operating state of the internal combustion engine.
Such exhaust-gas turbochargers, equipped with variable turbine geometry, are also used in particular in the overrun condition of the internal combustion engine. In overrun condition, the guide baffle is transferred into a damming or blocking position in which the inlet cross-section is markedly reduced, whereupon an increased exhaust-gas backpressure builds up in the line section upstream of the turbine, with the result that the exhaust gas flows with high speed through the channels between the guide vanes and the turbine wheel is acted upon with a high pulse. Due to the increased supercharger output, the combustion air supplied to the engine is also placed under an increased charge-air or boost pressure. Increased charge-air pressure acts upon the cylinder on the intake side. At the same time, an increased exhaust-gas backpressure is applied on the outlet side which counteracts the blow-off of the air compressed in the cylinder via brake valves into the exhaust-gas tract. In engine braking or exhaust braking operation, the piston in the compression stroke and exhaust stroke must perform compression work against the high overpressure in the exhaust-gas tract. A strong braking action is achieved by this means.
The desired high braking power can only be achieved, however, if a desired pressure distribution prevails within the turbine, and the exhaust gas flows through the turbine in the manner provided. Problematic in this context is, in particular, incorrect air flow which can occur owing to component and manufacturing tolerances, as well as due to wear and tear and heat-caused expansion, and which can severely impair a setpoint pressure characteristic within the turbine which has a negative effect not only on the engine braking performance, but also on the engine performance in the functioning drive operating mode. However, incorrect air flow can also develop through gaps which are subject to design and which are necessary for the movement of the guide vanes of the guide baffle of the variable turbine geometry in one of the flow-inlet cross-sections.
It is an object of the present invention to increase the efficiency of exhaust-gas turbines having variable turbine geometry. It is another object of the present invention to improve the supercharger performance particularly in engine braking operation, but also possibly in the functioning drive operating mode.
According to the exhaust-gas turbocharger of the present invention, the position of the flow ring in the housing of the supercharger may be variably adjusted. Conventionally, this flow ring is always constructed as a member fixedly joined to the supercharger housing, whereas according to the present invention, the flow ring is arranged to be movable. This provides the possibility that gap dimensions which are dependent on design or develop due to wear and tear, heat expansion or other causes may be reduced and possibly completely eliminated by a movement of the flow ring. Incorrect air flow may be largely or completely eliminated. Within the turbine, a desired pressure distribution may be adjusted which produces a desired flow of exhaust gas onto the turbine wheel. For example, when working with semi-axial/radial combination turbines, it is possible to construct a guide baffle, arranged in the semi-axial flow-inlet cross-section, with stationary vanes, and to configure this semi-axial guide baffle especially for the requirements in engine braking operation. In this construction, the aim is to reduce the radial flow-inlet cross-section to the greatest extent possible in engine braking operation, i.e., to close the radial guide baffle. However, in order to be able to adjust the radial guide vanes, a minimum gap may be necessary at the axial end faces of the radial guide vanes. To adjust the radial guide vanes, the adjustable flow ring may be shifted into a position further distant from the radial guide baffle. To close air gaps, the flow ring is subsequently pushed until contact at the end face of the radial guide vanes or of another structural element of the radial guide baffle.
The flow ring may be configured to be axially displaceable, thereby making it possible in particular to reduce guide-vane gaps at the radial guide baffle. Alternatively or in addition, however, it may also be possible to provide radial adjustability of the flow ring which may be attained, for example, by an eccentric shift of the flow ring and/or by a radial widening or tapering of the flow ring.
In the case of an axially displaceable flow ring, the shifting movement may be limited by stops which, in particular, limit the opening of a guide-vane gap of the radial guide baffle to a predefined dimension. This permitted axial path, which may be identical with the axial play of the flow ring, may be approximately 0.15 mm to 0.3 mm. This comparatively small dimension is intended to ensure that in the event of a malfunction, for example, in the event a control element adjusting the flow ring fails, the maximum play of the flow ring is limited to a dimension which may ensure functioning of the exhaust-gas turbocharger both in engine braking operation and in functioning drive operating mode.
In the case of a radial guide baffle having adjustable guide vanes, they may be supported, in each case via an axial shaft, not only on the supercharger housing, but also, e.g., in the displaceable flow ring. In the case of a double-sided support of the guide vanes which is provided in the flow ring as well, recesses may be provided in the flow ring to receive the allocated vane shaft, the depth of the recesses being, e.g., adapted to the axial length of the vane shafts in order to be able to accommodate the vane shafts even in the event the guide-vane gap is completely closed.
Optionally, it may also be possible in certain operating states of the internal combustion engine in the engine braking operation and/or in the functioning drive operating mode to provide a desired gap dimension with which the flow ratio and compression ratio within the supercharger housing in the turbine may be purposefully influenced in a specific manner. Furthermore, it may be possible to provide additional criteria for the adjustment of the flow ring, e.g., such that the flow-inlet cross-section for the radial and/or the semi-axial inflow may not exceed a maximum.
Further aspects and example embodiments of the present invention may be gathered from the figures and the following description.